The present invention relates to a control apparatus for a variable valve actuation system for an internal combustion engine.
An automobile engine provided with a variable valve actuation mechanism capable of changing a valve actuation state has been proposed. Changing a valve actuation state refers to changing a quantity of a valve state of an intake valve or an exhaust valve. Specifically, by changing the valve actuation state such as a valve working angle, a valve lift amount, and valve timing in accordance with the operating state of an automobile, the output properties of the engine can be changed. Such a variable valve actuation mechanism is operated by an oil control valve (OCV), a direct-current servomotor, a stepping motor, or a solenoid. Also, the OCV itself is controlled by a direct-current servomotor. Further, a mechanism that directly drives a valve electromagnetically without the use of a cam has also been proposed.
In any case, since the operating state of an automobile changes every moment, the control accuracy of such a variable valve actuation mechanism exerts a great influence on the output properties of an internal combustion engine, and the accuracy of control decreases. The decrease in control accuracy deteriorates the drivability of an automobile and the controllability of air-fuel ratio of the engine.
To increase this accuracy, feedback (FB) control has conventionally been carried out in which the quantity of the valve state, which is position information, is detected, and the detected quantity of the valve state is fed back to correct a deviation of an actual value from a target value.
Also, according to feedforward (FF) control, since the control is carried out by allowing for a change in a control amount in advance, anticipated control is carried out without a delay in control. For example, Patent Document 1 (Japanese Laid-Open Patent Publication No. 2002-364434) discloses control concerning a displacement process from one displacement end to the other displacement end of a valve.
In the conventional feedback control, as shown in FIG. 10(a), a control output is provided in response to a deviation of an actual value (indicated by a broken line) from a target value (indicated by a solid line). However, if the gain of control output is too small, there arises a problem in that the actual value always delays with respect to the target value, which causes a control delay. On the other hand, as shown in FIG. 10(b), if the gain is large, though the response is quick, overshoot may occur, and further the control output oscillates, so that the control becomes unstable. Also, in particular, as shown in FIG. 10(c), for example, if the vehicle speed is different, the control output becomes unstable even if the gain is the same in the case where the response characteristic changes in accordance with the quantity of state. Therefore, a further larger gain cannot be provided. In such a case, it is considered that gain scheduling in which the gain is changed in accordance with control parameters is effective. However, the design method therefor has not been clear. In the graphs of FIGS. 10(a) to 10(c), the abscissas represent time, and the ordinates represent the magnitude of control amount or control output.
Also, a control system may be considered in which a map is prepared in advance and feedback control is carried out together with feedforward control. However, the method for carrying out this control system and the design method therefor are not clear. In particular, the control based on only a deviation of an actual quantity of the valve state from a target quantity of the valve state does not take into account the changing an operating state of an automobile fully enough to the degree that the operating state of the automobile can be responded quickly and accurately. The control method described in Patent Document 1 is control concerning only a displacement process from one displacement end to the other displacement end of a valve, so that this control method cannot solve these problems.